What does the manual say? For a normally aspirated engine, unless there are specific limits stated, there should be no problem reducing RPM, even at max throttle. The engine power output is limited by RPM as much as it is by manifold pressure, so it cannot put out full power at reduced RPM.
Running slightly "oversquare" is not inherently a problem. It is actually more efficient to run at wide open throttle, with power limited by RPM. If within the mfgr's recommended envelope, then do it!